Experiments are presented which examine the interaction between the retina and the pigment epithelium. In particular, we will investigate the molecular biology of the balanced process of outer segment renewal and shedding which is responsible for maintaining the outer segments at a constant length. The overall goal of this application is to better understand the regulation of renewal, shedding, and phagocytosis in frogs maintained on a controlled diurnal lighting cycle, and to describe the effect of alterations in the normal cycle. Using the frog as a model and the techniques of microscopy, biochemistry, and autoradiography, we will examine in detail outer segment renewal, shedding and phagocytosis. Specifically, in both in vivo and in vitro experiments, renewal and shedding will be studied in order to determine: 1) what primes photoreceptors to shed, 2) how does light induce shedding, 3) what is the primary site of light reception, 4) does the neuroendocrine system play a role in renewal and shedding, 5) what molecular events occur after light reception which finally result in outer segment shedding, 6) during shedding, what changes occur which make the shed tip recognized and phagocytized by the pigment epithelium, 7) what morphological and biochemical changes occur within the pigment epithelium during shedding and phagocytosis of outer segment tips, and 8) what is the role of phospholipids, particularly phosphatidyl choline and phosphatidyl inositol, in these processes? These experiments should provide important information on the normal function of the retina and pigment epithelium, and may give us further insight into how these two tissues operate together to perform the complex metabolic and physiological functions which characterize a normal, healthy retina.